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MBC in Press, published online ahead of print July 19, 2005
Mol. Biol. Cell 10.1091/mbc.E05-03-0252

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Submitted on March 25, 2005
Revised on June 30, 2005
Accepted on July 11, 2005

Myosin-Va Regulates Exocytosis through the Submicromolar Ca2+-dependent Binding of Syntaxin-1A

Michitoshi Watanabe,*{dagger}{ddagger} Kazushige Nomura,*{ddagger}{sect}|| Akihiro Ohyama,{ddagger}{sect}¶# Ryoki Ishikawa,@ Yoshiaki Komiya,{sect} Kohei Hosaka,** Emiko Yamauchi,{dagger}{dagger} Hisaaki Taniguchi,{dagger}{dagger} Nobuyuki Sasakawa,{ddagger}{ddagger} Konosuke Kumakura,{ddagger}{ddagger} Tatsuo Ushiki,{sect}{sect} Osamu Sato,||||¶¶ Mitsuo Ikebe,|||| and Michihiro Igarashi*{dagger}

Divisions of *Molecular and Cellular Biology and {sect}{sect}Microscopic Anatomy and Bio-Imaging, Graduate School of Medical and Dental Sciences and {dagger}Center for Trans-disciplinary Research, Niigata University, Niigata, Niigata 951-8510, Japan; Departments of {sect}Molecular and Cellular Neurobiology, ||Orthopedic Surgery, Anesthesiology and Reanimatology, and @Molecular and Cellular Pharmacology, Gunma University School of Medicine, Maebashi, Gunma 371-8511, Japan; **Basic Sciences for Medicine, Gunma University School of Health Sciences, Maebashi, Gunma 371-8514, Japan; {dagger}{dagger}Division of Enzyme Physiology, Institute for Enzyme Research, University of Tokushima, Tokushima, Tokushima 770-8503, Japan; {ddagger}{ddagger}Life Science Institute, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan; and ||||Department of Physiology, University of Massachusetts Medical School, Worcester, MA 01655-0127

Monitoring Editor: Anthony Bretscher

Myosin-Va is an actin-based processive motor that conveys intracellular cargoes. Synaptic vesicles are one of the most important cargoes for myosin-Va, but the role of mammalian myosin-Va in secretion is less clear than for its yeast homologue, Myo2p. In the current studies, we show that myosin-Va on synaptic vesicles interacts with syntaxin-1A, a t-SNARE involved in exocytosis, at or above 0.3 µM Ca2+. Interference with formation of the syntaxin-1A-myosin-Va complex reduces the exocytotic frequency in chromaffin cells. Surprisingly, the syntaxin-1A-binding site was not in the tail of myosin-Va but rather in the neck, a region that contains calmodulin-binding IQ-motifs. Furthermore, we found that syntaxin-1A binding by myosin-Va in the presence of Ca2+ depends on the release of calmodulin from the myosin-Va neck, allowing syntaxin-1A to occupy the vacant IQ-motif. Using an anti-myosin-Va neck antibody, which blocks this binding, we demonstrated that the step most important for the antibody’s inhibitory activity is the late sustained phase, which is involved in supplying readily releasable vesicles. Our results demonstrate that the interaction between myosin-Va and syntaxin-1A is involved in exocytosis and suggest that the myosin-Va neck contributes not only to the large step size but also to the regulation of exocytosis by Ca2+.

{ddagger}These authors contributed equally to this work.

Present addresses: #Ophthalmology and ¶¶Pharmacology, Juntendo University School of Medicine, Hongo 2-1-1, Bunkyo-ku, Tokyo 113-8421, Japan.

Address correspondence to: Michihiro Igarashi (tarokaja{at}med.niigata-u.ac.jp)




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